Diffraction gratings are used to separate the various channels in wavelength division multiplexing (and de-multiplexing) applications according to wavelength. Various free space diffraction grating designs are known in the art. Presently, the performance of these diffraction gratings is limited by (high) polarization dependent loss (PDL) and (low) diffraction efficiency, due in part to manufacturing tolerances. The reduction of the polarization dependence can be achieved by using deeper gratings, either in reflection or transmission, but is limited when higher-incident angles are required to increase the dispersion.
The next generation of high performance telecommunication systems will require diffraction efficiency greater than 90%, polarization dependent loss less than 0.25 dB, uniformity of diffraction efficiency better than 5% over the wavelength range of 1300 to 1600 nm, ease of manufacture, and out of plane geometry, at lower cost than what is currently available.
As used herein, WDM (wavelength division multiplexing) will be used to describe any of the various wavelength selective devices/systems used for the separation or combination of optical communication channels of different wavelengths, including DWDM (dense wavelength division multiplexing), WSS (wavelength selective switch), CWDM (coarse wavelength division multiplexing), etc.
The general purpose of a diffraction grating is to disperse (or bring together) light as a function of wavelength and angle. The basic equation that describes this behavior is:mλ/d=sin(α)+sin(β),  (1)where m is the diffraction order, λ is the wavelength of interest, d is the periodic groove spacing of the grating, α is the angle of incidence of the light onto the grating relative to the grating surface normal, and β is the angle of diffraction of the light leaving the surface of the grating relative to the grating surface normal.
The amount of dispersion, which here is defined as the change in diffraction angle per change in wavelength, is described by the equation:dβ/d λ=m/(d cos(β)),  (2)where the definition of terms is the same as for equation 1.
The light that is input to a grating may be of a single polarization or a combination of two polarizations (e.g., orthogonal polarizations). The orthogonal polarizations are called TE (transverse electric) and TM (transverse magnetic). In a typical optical telecommunication wavelength channel separation or combination system, the light that is separated or combined is referred to as de-multiplexed or multiplexed, respectively. For purposes of this invention description, de-multiplexing systems will be described, although the embodiments of the invention are intended to also cover multiplexing systems, as one skilled in the art will appreciate.
Light exiting a fiber optic cable is a combination of TE (P-polarized) and TM (S-polarized) light. The particular polarization state (TE or TM) may vary randomly in time. Thus in order to obtain all of the information being carried by the light intensity via the fiber optic cable, both polarizations, which together carry the data being transmitted, must be utilized. Traditional diffraction gratings generally possess different diffraction efficiencies for the different polarization states of the incident light, which can lead to a loss of information. The measure of this loss is called the polarization dependent loss (PDL) and is defined to be proportional to the ratio of the diffraction efficiencies for the separate polarizations expressed in decibels (dB). A low PDL is required for WDM-type systems to maintain a low error rate and high information fidelity.
FIG. 1 illustratively shows a triangular groove, ruled diffraction grating as known in the art, where reference numeral 1 is the substrate, usually glass, that supplies a rigid surface upon which the ruled grating structures are located. Reference numeral 2 is the periodic groove structure, shown in cross section and illustrating only a few of the grooves, which in this illustrative example are metal coated. The incident light to be diffracted is shown by reference numeral 3. Reference numerals 4 and 5 represents the diffracted light, which is separated in angle by wavelength, where reference numeral 4 represents a wavelength λ1 that is different than λ2 shown by reference numeral 5.
Angular dispersion, as described by equation 2, can be measured for a typical grating, as might be employed, e.g., in a WDM-type application, to be dβ/dλ=m/(d cos(β))=1/(1.01*0.73)=1.24 rad/μm. This value was calculated for a 900 groove/mm grating, or equivalent, at 1.5 micron wavelength in Littrow mount (i.e., angle of incidence equals angle of diffraction). For this particular example, β was approximately 43 degrees. FIG. 2 shows the diffraction efficiency of TE polarized light 2 and TM polarized light 1 for the ruled grating of FIG. 1. Reference numeral 3 represents the polarization dependent loss (PDL) as calculated from the ratio of the TE- and TM-polarized diffracted light. The PDL for such a grating, coupled with the low diffraction efficiency and low angular dispersion, combine to limit their usefulness for telecom applications. State of the art systems require higher angular dispersion, with nearly zero PDL, coupled with higher diffraction efficiency.
When higher angular dispersion is required, for example, to reduce the size of the WDM-type device, systems may typically utilize a sinusoidal diffractive groove shape. FIG. 3 represents a prior art sinusoidal surface relief diffraction grating as seen in cross section, similar to FIG. 1, except that the groove structure is sinusoidal instead of triangular. In this case, as in that of the ruled grating, the grooves are invariant in one direction and periodic in the orthogonal direction. The substrate is again represented by reference numeral 1, while the sinusoidal groove structure is represented by reference numeral 2. The incident light 3, is diffracted and separated into light of different wavelengths, 4 and 5, according to the grating equation. The angles used to calculate the diffraction directions are measured, as in all cases, relative to the grating normal 6.
FIG. 4 illustrates a typical plot 41 of diffraction behavior, representing diffraction efficiencies for TE-(43) and for TM-(42) polarizations for a sinusoidal grating as shown in FIG. 3. In this exemplary illustration, the period is d=0.909 microns and the PDL is greater than 0.7, which is greater than the requirement for gratings used in WDM-type devices. The PDL of such a grating is greater than 1.0 dB for TM efficiencies above 90%. TE efficiency is less than 40%; consequently, additional optics are required to convert TE polarized light from the fiber optic transmission system into TM polarization, where after the diffraction grating can disperse it into the various channels required. The optical components that convert one polarization into the opposite polarization add additional losses and costs to the system.
When higher angular dispersion is required, larger angles of diffraction are required according to equation 2. The difference between the efficiencies in TE and TM polarization then becomes much more significant. An example for a ID sinusoidal gold grating with period d=1.2056 μm that works around Littrow mount at wavelength 1.55 μm is shown with regard to FIGS. 5(a-c). FIG. 5a shows the efficiency in order 1 at light incidence equal to 40°. The groove depth dependence in FIG. 5a shows the possibility to obtain equal TE and TM efficiency with relatively deep grooves (groove depth-to-period ration h/d=1.128). FIG. 5b shows the spectral dependence for h/d=1.128 and parameters from (a). Although the spectral dependence behaves in an opposite manner in both polarizations, overall acceptable results might be achieved within the working spectral interval. FIG. 5c shows the groove depth dependence for an incidence of 60° and period d=0.89489 μm. As observed, when higher angular dispersion is required (shorter period and larger incident angle), it is not possible to achieve sufficiently high enough in the TE polarization.
Currently deployed prior art WDM-type systems are single use, meaning that the optical system utilizes one input and one output for each diffraction grating. Such a WDM-type system, in this case a de-multiplexer 600-1, is illustrated in FIG. 6. The input fiber optic cable 61 emits light 62 that is collimated by the lens 63. The light, which contains elements of both TE and TM polarization, passes through a device 64, which converts TE polarization into TM while not changing the incident TM polarization. This allows the diffraction grating 65 to see only a single polarized light where the diffraction efficiency is relative high, such as the grating of FIG. 3, whose efficiency is shown in FIG. 4 as reference numeral 42. The diffracted light 66 is separated by wavelength into different angles, passes through a lens 67, and is brought to focus at 68 for analysis, switching, or further transmission, to name a few of the many results of separating the optical communication channels. To those skilled in the art, such a system can be used in the reverse direction and operated as a multiplexer to combine many wavelengths onto the same fiber. This would require the input to be reference numeral 68, the polarization rotator device 64 to be placed adjacent to 67, and the output would be into 61.
In order to shrink the size, reduce the cost, and achieve desired performance, telecommunication systems will require higher groove frequencies with substantially equal diffraction efficiency for both TE and TM polarizations.